While everyone knew that Windows 7 would be a commercial success because of Microsoft’s massive OEM program, most people didn’t think that it would experience such rapid adoption. In fact, preliminary reports suggested that Windows Vista’s rollout in 2007 was more successful than last year’s Windows 7 launch.

Market share data soon proved that assertion wrong, though. Windows 7 sales remained strong throughout the holiday season, and the OS surged past OSX in January. In fact, according to the same Webmasterpro data, Windows 7 already has more than half of Vista’s market share.

Interestingly, it doesn’t seem like users are upgrading from Vista in droves, though. Since 7’s launch in October, Vista market share has declined only a few points. It seems that the majority of Windows 7’s growth has come from XP users upgrading or junking old machines, because XP’s market share has declined by roughly 10% since 7’s release.

In the end, it represents a massive turnaround from 12 months ago. With Vista’s miserable performance in the marketplace and Mac OSX’s surging market share, many pundits crowed that Microsoft was on the decline. Now Microsoft looks like it’s returned to form. Steve Ballmer finally has something to get excited about.